The two main languages supported by Unity are Javascript and C# (Boo is also supported, but no one uses Boo at all, and as a programming language it is completely ridiculous). Your question about which one is more scalable is really irrelevant, because both languages run off the same API, specifically built for Unity, and they both run off the same function calls, and thus they are equally powerful.

I'd have to go with Javascript as the best language for a beginner. Most of Unity's Scripting Documentation and helpfiles are written specifically for someone programming in Javascript. We are currently using Javascript for Reggae Speed, simply because it is simpler. Although C# is another good option, C# has all these weird rules and restrictions(like the Class name has to be the same as the script name, and they all have to inherit MonoBehaviour from the object pool, and other such irritating things ). However, although it might be easier to use Javascript, and though it makes sense to use it as a beginner to Unity, C# does have it's benefits. The reason that we used C# in our previous MMO project was because that C#s .NET framework is much easier to integrate with Server Systems, Databases and Hashtables. Writing it in C# also makes it easier to port the Unity Game to other APIs and systems, such as XNA (which is primarily C# based).

So all in all, I would recommend Javascript for an absolute beginner, but C# can also be very useful for Multiplayer Programming and some of the high-end stuff. In the long run, I recommend learning both .